WHAT ARE LANDSLIDES?

What Are Landslides?

Landslides are rock, earth, or debris flows
on slopes due to gravity. They can occur on any terrain given the right conditions of
soil, moisture, and the angle of slope. Integral to the natural process of the
earth's surface geology, landslides serve to redistribute soil and sediments in a process
that can be in abrupt collapses or in slow gradual slides. Such is the nature of the
earth's surface dynamics. Also known as mud flows, debris flows, earth failures,
slope failures, etc., they can be triggered by rains, floods, earthquakes, and other
natural causes as well as human-made causes, such as grading, terrain cutting and filling,
excessive development, etc. Because the factors affecting landslides can be
geophysical or human-made, they can occur in developed areas, undeveloped areas, or any
area where the terrain was altered for roads, houses, utilities, buildings, and even for
lawns in one's backyard. They occur in all fifty states with varying frequency and more
than half the states have rates sufficient to be classified as a significant natural
hazard.

The U.S. Geological Survey, working with other federal
agencies, has efforts underway to study, plan, and mitigate landslide risks. So have some
communities across the country. Many deal with landslides as part of flood control,
erosion control, hillside management, earthquake hazard mitigation, road stabilization,
and other programs.

Perhaps the most common reminders of
landslide risks are those "Watch For Falling Rocks" highway signs. Although
"sliding rocks" is more apt, very few get to see a land slide. Occasionally we
see small rocks or debris on the pavement, but a large size slide usually starts with such
small incidents. Visually, a landslide resembles a snow avalanche, only with a louder
rumbling noise, and is capable of generating enough force and momentum to wipe anything in
its path. One such devastating landslide wiped entire towns and villages in Columbia in
1985 when 20,000 died.

The pictures you
see on this web site (including the background of this page), are recent examples from
around the country. They show what's left after a slide. In some cases, only the rail or
pavement is mangled, in others a house or building crushed, but in almost every aftermath,
the losses are real, the damages total, and the terrain changes permanent.

Landslides cause one to two billion dollars
in damage each year in the US and claim as many as fifty lives. That's more
devastating than all the other natural hazards combined. They affect utilities,
transportation, and all other forms of infrastructure, whether public or private.

As development
pressures around the country increase, so does the likelihood of building in areas
susceptible to landslides. Such areas are neither isolated nor far in-between. They span
the entire eastern part of US, from New England to the Appalachian region encompassing
some of the most scenic areas in the east as well as large urban areas. Landslide risks
loom through them all. Pittsburgh and Cincinnati are two examples of urbanized areas
with frequent landslides where developments on hills and hillsides are common. In the
Great Plains, heavy rains combined with loss of vegetation due to wildfires trigger
landslides in clay-rich rocky areas. On the west coast, earthquakes add to the causes of
landslides. For example, the 1994 Northridge earthquake triggered many thousand landslides
in the Santa Susanna Mountains. In short, no region of the country is safe from
landslides, whether caused by geophysical or human-made factors.

Although the term landslide is often used
somewhat loosely to mean any fairly rapid movement of rocks and sediment downslope, it is
actually more accurate to use the term mass wasting to refer to the wide variety of mass
movement processes that wear away at the Earth's surface.

What Factors Affect The Mass
Wasting Process?

There are three main factors that
control the type and rate of mass wasting that might occur at the Earth's surface:

Slope gradient: The steeper the slope of the land, the more likely that mass
wasting will occur.

Slope consolidation: Sediments and fractured or poorly cemented rocks and
sediments are weak, and more prone to mass wasting.

Water: If slope materials are saturated with water, they may lose cohesion and
flow easily.

There are three basic types of mass
wasting:

While driving your vehicle through mountain
country, such as parts of eastern California, you may have noticed signs such as: WARNING!
ROCK FALL HAZARD.

Because weathering is an ongoing process,
steep mountain slopes are constantly wasting away, often in the form of rocks falling and
bouncing down slopes. Such falls can be triggered by freezing of water, the growth of
plants (and their roots), earthquakes, or by people hiking on the slope. Rock falls occur
in just a matter of seconds, so they are difficult (though fun) to observe. But, you can
tell where rock falls occur on a mountain slope by looking for talus, a buildup of loose,
angular rocks at the base of a steep slope.

Typically cone-shaped, these piles of rock
debris are found at the base of the San Gabriel and San Bernardino Mountain systems, the
former almost within view of the 210 freeway (on a clear day, of course!).

For those of you who may have some difficulty
in finding the talus slopes, take a look below:

Whenever a mass of slope material moves as a
coherent block , we say that a slide has taken place. There are several types of slides,
but one of the most common is a slump. A slump occurs when a portion of hillside moves
downslope under the influence of gravity. A slump has a characteristic shape, with a scarp
or cliff at the top of the slump, and a bulge of material (often called the toe of the
slump) at the base of the slump. In the image shown below, see if you can spot the
location of the scarp and toe of the slump.

What is particularly interesting about this
slump picture is that it was taken in the year 1907 in the Berkeley Hills just east of San
Francisco. Needless to say, this area has now been built up. Houses probably now stand
where the slump took place. Question: Do you think that the danger of slumping is over for
the Berkeley Hills region, now that it has been built up? Has the danger perhaps
increased? What factors might increase the risk of slumping in areas such as the Berkeley
Hills, or the Palos Verdes Peninsula and Pacific Palisades regions?

For those of you wondering where the scarp
and toe of the slide might be, check out the image shown below.

We say that a flow has occurred if the
material moving downslope is being transported as a very thick fluid (like a river of
debris, rock, and/or soil), rather than as a coherent unit. Often, water is the primary
transport agent for the flow.

Lahars

For sheer drama and destruction, you can't
beat the mud flows that can occur when a volcano erupts. These flows are called lahars.
After the Mount St. Helens eruption, for example, the heat of the volcanic products melted
the snowcap. The resulting liquid water rushed down the volcano flanks, incorporating
debris as it progressed. The damage to forests and humans was extensive.

Look at that mudline -- up to
the window!

Imagine what would have happened, though, if
Mt. St. Helens had been closer to major urban centers, such as Seattle, Washington, or
Portland, Oregon. The eruption at Mount Pinatubo, in the Philippines, was an instance
where a volcano erupted close to a densely populated area. Luckily, geologists had
predicted that the eruption would occur, and were able to advise a timely evacuation of
the region.

In this aerial view, you can see the
destruction of a bridge by a lahar from Mount Pinatubo. The white dots, by the way, are
humans crossing makeshift bridges over the river.

Become familiar with the land around you.
Learn whether landslides or debris flows have occurred in your area by contacting local
officials, state geological surveys or departments of natural resources, USGS maps, and
university departments of geology. Slopes where landslides or debris flows have occurred
in the past are likely to experience them in the future.

Support your local government in efforts to
develop and enforce land-use and building ordinances that regulate construction in areas
susceptible to landslides and debris flows. Buildings should be located away from known
landslides, debris flows, steep slopes, streams and rivers, intermittent-stream channels,
and the mouths of mountain channels.

Watch the patterns of storm-water drainage on
slopes near your home, and note especially the places were runoff water converges,
increasing flow over soil-covered slopes. Watch the hillsides around your home for any
signs of land movement, such as small landslides or debris flows or progressively tilting
trees.

Contact your local authorities to learn about
the emergency response and evacuation plans for your area, and develop your own emergency
plans for your family and business.

Stay alert and stay awake! Many landslide and
debris flow fatalities occur when people are sleeping. Listen to a radio for warnings of
intense rainfall. Be aware that intense short bursts of rain may be particularly
dangerous, especially after longer periods of heavy rainfall and damp weather.

Listen for any unusual sounds that might
indicate moving debris, such as trees cracking or boulders knocking together. A trickle of
flowing or falling mud or debris may precede larger landslides. If you are near a stream
or channel, be alert for any sudden increase or decrease in water flow. Such changes may
indicate landslide activity upstream, so be prepared to move quickly. Don't delay! Save
yourself, not your belongings.

If you are in areas susceptible to landslides
and debris flows, consider leaving if it is safe to do so. If you remain at home, move to
a part of the house farthest away from the source of the landslide or debris flows, such
as an upper floor, but keep an escape route open should it become necessary to leave the
house.

Be especially alert when driving. Embankments
along roadsides are particularly susceptible to landslides. Watch the road for collapsed
pavement, mud, fallen rocks, and other indications of possible landslides or debris flows.

Keep looking for signs that the land is
moving. Landslides can occur weeks or months after intense storms.

DID YOU KNOW...

The most expensive landslide in U.S. history
occurred in Thistle, Utah, in spring, 1983. It reached 1/2 miles from top to bottom
and ranged in width from 1,000 feet to about 1 mile. Total costs attributable to the
landslide exceeded $500 million.

Landsliding in the United States is estimated
to cause an annual loss of about $1.5 billion and at least 25 fatalities.

The Loma Prieta earthquake in October 1989
triggered thousands of landslides throughout an area of 5,400 square miles. In addition to
causing at least tens of millions of dollars of damage to houses, other structures, and
utilities, landslides blocked many transportation routes, greatly hampering rescue and
relief efforts.

Mudflows tend to flow in channels, but will
often spread out over a floodplain. They generally occur in places where they have
occurred before.

FACT SHEET: LANDSLIDES AND
MUDFLOWS

Landslide and mudflows usually strike without
warning. The force of rocks, soil, or other debris moving down a slope can devastate
anything in its path. Take the following steps to be ready.

BEFORE

Get a ground assessment of your
property.

Your county geologist or county planning
department may have specific information on areas vulnerable to landsliding. Consult a
professional geotechnical expert for opinions and advice on landslide problems and on
corrective measures you can take.

Minimize home hazards.

Plant ground cover on slopes and build
retaining walls.

In mudflow areas, build channels or deflection
walls to direct the flow around buildings.

Remember: If you build walls to divert debris
flow and the flow lands on a neighbor's property, you may be liable for damages.

Learn to recognize the landslide warning
signs.

Doors or windows stick or jam for the first
time.

New cracks appear in plaster, tile, brick, or
foundations.

Outside walls, walks, or stairs begin pulling
away from the building.

Slowly developing, widening cracks appear on
the ground or on paved areas such as streets or driveways.

Underground utility lines break.

Bulging ground appears at the base of a slope.

Water breaks through the ground surface in new
locations.

Fences, retaining walls, utility poles, or
trees tilt or move.

You hear a faint rumbling sound that increases
in volume as the landslide nears.The ground slopes downward in one specific direction and
may begin shifting in that direction under your feet.

Make evacuation plans. Plan at least
two evacuation routes since roads may become blocked or closed.

Develop an emergency communication plan.
In case family members are separated from one another during a landslide or mudflow
this is (a real possibility during the day when adults are at work and children are at
school), have a plan for getting back together.

Ask an out-of-state relative or friend to
serve as the "family contact". After a disaster, it's often easier to call long
distance. Make sure everyone knows the name, address, and phone number of the contact
person.

DURING

If inside a building:

Stay inside.

Take cover under a desk, table, or other piece
of sturdy furniture.

If outdoors:

Try and get out of the path of the landslide
or mudflow.

Run to the nearest high ground in a direction
away from the path.

If rocks and other debris are approaching, run
for the nearest shelter such as a group of trees or a building.

If escape is not possible, curl into a tight
ball and protect your head.

Sinkholes

A sinkhole occurs when groundwater dissolves
a vulnerable land surface such as limestone, causing the land surface to collapse from a
lack of support. In June 1993, a 100-foot wide, 25-foot deep sinkhole formed under a
hotel parking lot in Atlanta, killing two people and engulfing numerous cars.

AFTER

Stay away from the slide area.
There may be danger of additional slides.

Check for injured and trapped
persons near the slide area. Give first aid if trained.

Remember to help your neighbors who may
require special assistance--infants, elderly people, and people with disabilities.

Listen to a battery-operated radio or
television for the latest emergency information.

Remember that flooding may occur after a
mudflow or a landslide.

Check for damaged utility lines. Report
any damage to the utility company.

Check the building foundation, chimney, and
surrounding land for damage.

Replant damaged ground as soon as possible
since erosion caused by loss of ground cover can lead to flash flooding.